The present invention relates to an optical device used in a communication system using optical fibers and particularly relates to an optical device using a gradient index rod lens provided with a reflecting layer.
In recent years, increase in capacity of an optical fiber communication network has been strongly demanded because of the rapid advance of popularization of Internet. Development of wavelength division multiplexing (WDM) communication as a method for increasing the capacity has been advanced rapidly. In WDM communication, light beam components with slightly different wavelengths are demodulated individually and mixed into an optical signal so that the optical signal can be transmitted through one optical fiber. At an end point of transmission, the optical signal in which the light beam components with different wavelengths are mixed is separated into light beam components by wavelengths so that the light beam components with different wavelengths can be received. The mixing of light beam components into an optical signal is referred to as xe2x80x9cmultiplexingxe2x80x9d and the separation of the optical signal into light beam components is referred to as xe2x80x9cdemultiplexingxe2x80x9d. A multiplexer/demultiplexer using an optical filter is used as a method for performing such multiplexing/demultiplexing.
An upper half of FIG. 6 shows an example of the multiplexer. As shown in the upper half of FIG. 6, the wavelength multiplexer has three optical fibers, a pair of lenses, and a filter. That is, light with a wavelength of xcex1 is output from an optical fiber 101. The light is input to a rod lens 103. The light with a wavelength of xcex1 reaches a filter 104 while converted into a parallel beam by the rod lens 103. The filter 104 reflects the light with a wavelength of xcex1. The reflected light is input to the rod lens 103 again and converged by the rod lens 103. The converged light is coupled to an optical fiber 102. On the other hand, light with a wavelength of xcex2 is output from an optical fiber 111. The light with a wavelength of xcex2 reaches the filter 104 while converted into a parallel beam by a rod lens 113. The filter 104 transmits the light with a wavelength of xcex2. The light transmitted through the filter 104 is input to the rod lens 103 and converged by the rod lens 103. The converged light is coupled to the optical fiber 102. In this manner, a light component with a wavelength of xcex1 output from the optical fiber 101 and a light component with a wavelength of xcex2 output from the optical fiber 111 are multiplexed so that the multiplexed light is coupled to the optical fiber 102.
Demultiplexing is performed as shown in a lower half of FIG. 6. That is, light components with wavelengths of xcex1 and xcex2 are output from the optical fiber 102. The light components are input to the rod lens 103. The light components reach the filter 104 while converted into parallel beams by the rod lens 103. The filter 104 reflects the light component with a wavelength of xcex1. The reflected light component is input to the rod lens 103 again and converged by the rod lens 103. The converged light component is coupled to the optical fiber 101. On the other hand, the light component with a wavelength of xcex2 reaches the filter 104 while converted into a parallel beam by the rod lens 103. The filter 104 transmits the light component with a wavelength of xcex2. The light component transmitted through the filter 104 is input to the rod lens 113 and converged by the rod lens 113. The converged light component is coupled to an optical fiber 112. In this manner, light components with wavelengths of xcex1 and xcex2 output from the optical fiber 102 are demultiplexed into the optical fibers 101 and 112.
When the optical system shown in FIG. 6 is used practically, a filter 4 may be brought into contact with an end surface 43b of a left rod lens 3 as shown in FIG. 7. Incidentally, a right rod lens is not shown in FIG. 7. When the filter 4 is disposed as shown in FIG. 7, it is unnecessary to position and fix the rod lens and the filter separately for forming the optical system as a module. There is an advantage in that the long-term stability of the optical system can be improved as well as the optical system can be assembled easily. This is a configuration effectively using the characteristic that the rod lens has a planar end surface.
In FIG. 7, an output optical fiber 1 and an input optical fiber 2 are disposed in parallel to each other, similarly to those in FIG. 6. End surfaces 41 and 42 of the two optical fibers 1 and 2 are disposed so as to face an end surface 43a of the rod lens 3 through a gap of a suitable distance. The gap may be formed as an air layer or may be filled with a medium 5 such as matching oil or an adhesive agent.
For example, the index distribution of the rod lens 3 is given by the following formula (see Japanese Patent Laid-Open No. 91316/1985):
n(r)2=n02xc2x7{1xe2x88x92(gxc2x7r)2+h4(gxc2x7r)4+h6(gxc2x7r)6+h8(gxc2x7r)8+ . . . }
in which r is a radial distance measured from the optical axis of the rod lens, r0 is the effective radius of the rod lens, n0 is the refractive index of the rod lens on the optical axis of the rod lens, g is a gradient index distribution coefficient of second order, and h4, h6, h8 . . . are gradient index distribution coefficients of fourth, sixth, eighth . . . order respectively.
The periodic length P of the rod lens is equal to 2xcfx80/g. When the length Z of the rod lens 3 on the optical axis of the rod lens 3 is set to be slightly smaller than 0.25P, a luminous flux output from the optical fiber 1 is collimated into approximately parallel light rays at the end surface provided with the filter 4. Hence, the luminous flux reflected by the filter 4 is converged again and returned to the optical fiber 2.
When the position of the end surface 41 of the optical fiber 1 is adjusted both in a direction of the optical axis 21 and in a direction perpendicular to the optical axis 21 while the two optical fibers 1 and 2 are disposed in parallel to the optical axis 23 of the rod lens 3, the luminous flux output from the optical fiber 1 is focused on the end surface 42 on the optical axis 22 of the optical fiber 2 so that high coupling efficiency can be obtained.
In the arrangement shown in FIG. 7, however, a principal beam component (defined as a beam component of maximum intensity forming a symmetrical center of Gaussian beams) of the light output from the optical fiber 1 disagrees with the optical axis 22 of the optical fiber 2. Hence, an inclination xcex8d is generated in an XZ plane as shown in FIG. 8. As a result, a coupling loss corresponding to the size of the inclination xcex8d is produced.
The inclination xcex8d can be eliminated if the length of the rod lens 3 is set to be 0.25 pitches while the end surfaces 41 and 42 of the two optical fibers are brought into contact with the end surface 43a of the rod lens. In such a design, there is however a disadvantage in that the degree of freedom for delicate adjustment of focusing and positioning the fibers is spoiled. If the lens length is shortened, the adjustment can be made easily because the distance between each of the end surfaces 41 and 42 of the optical fibers 1 and 2 and the end surface 43a of the lens 3 becomes long, but there is a problem that the loss due to the inclination xcex8d becomes large.
The invention is to provide a condition for suppressing the loss due to the inclination xcex8d to be in a practically allowable range.
An optical device includes an optical system constituted by a combination of an output optical fiber, an input optical fiber and a gradient index rod lens, the output optical fiber and the input optical fiber being arranged so that optical axes of the two optical fibers are parallel to each other with a distance of W, the gradient index rod lens having a length of Z on its optical axis and having a radially gradient index distribution represented by a formula:
n(r)2=n02xc2x7{1xe2x88x92(gxc2x7r)2+h4(gxc2x7r)4+h6(gxc2x7r)6+h8(gxc2x7r)8+ . . . }
in which r is a radial distance measured from the optical axis of the rod lens, r0 is the effective radius of the rod lens, n0 is the refractive index of the rod lens on the optical axis of the rod lens, g is a gradient index distribution coefficient of second order, and h4, h6, h8 . . . are gradient index distribution coefficients of fourth, sixth, eighth . . . order respectively, wherein:
the optical axes of the two optical fibers are disposed in parallel to the optical axis of the rod lens so that, after a luminous flux output from the output optical fiber is input to the rod lens through a first end surface of the rod lens and converted into an approximately parallel luminous flux in the inside of the rod lens, at least one part of the parallel luminous flux is reflected by a reflecting unit such as a filter disposed on a second end surface of the rod lens opposite to the first end surface of the rod lens, converted into a convergent luminous flux again in the inside of the rod lens, and input to the input optical fiber; and
the optical device preferably satisfies a condition:
Wxc2x7gxc2x7(0.25xe2x88x92Z/P)2xe2x89xa66xc3x9710xe2x88x925 
in which P is a periodic length of the rod lens (equivalent to 2xcfx80/g), and Z is the length of the rod lens on the optical axis of the rod lens.
More preferably, the condition range is:
Wxc2x7gxc2x7(0.25xe2x88x92Z/P)2xe2x89xa62xc3x9710xe2x88x925 
Preferably, the two optical fibers have end surfaces parallel to a line perpendicular to the optical axes of the two optical fibers and inclined at an angle of inclination to a plane perpendicular to the optical axes of the two optical fibers. Preferably, the rod lens has an end surface facing the two optical fibers, parallel to a line perpendicular to the optical axes of the two optical fibers and inclined at an angle of inclination to a plane perpendicular to the optical axes of the two optical fibers.
The present disclosure relates to the subject matter contained in Japanese patent application No. 2001-129344 (filed on Apr. 26, 2001), which is expressly incorporated herein by reference in its entirety.